Experiment 116.—Prepare a H generator, - a flask with a thistle- tube and a philosopher's lamp tube (Fig. 46), put in some granulated Zn, water, and HCl. Test the purity of the escaping gas (Experiment 23), and when pure, light the jet of H. H is now burning in air. To be sure that there is no As in the ingredients used, hold the inside of a porcelain evaporating-dish directly against the flame for a minute. If no silvery-white mirror is found, the chemicals are free from As. Then pour through the thistle-tube, while the lamp is still burning, 1cc.solution of AS2O3 in HCl or H2O a bit of As2O3 not larger than a grain of wheat in 10 cc. HCl.
See whether the color of the flame changes; then hold the evaporating-dish once more in the flame, and notice a metallic deposit of As. Set away the apparatus under the hood and leave the light burning.
This experiment must not be performed unless all the cautions are observed, since the gas in the flask (AsH3) is the most poisonous known, and a single bubble of it inhaled is said to have killed the discoverer. By confining the gas inside the flask there is no danger.
Instead of using As2O3 solution, a little Paris green, wall paper suspected of containing arsenic, green silk, or green paper labels, etc., may be soaked in HCl, and tested.
209. Explanation.—The chemical changes are as follows: The compounds of As, in this case As2O3, in presence of nascent H, are immediately converted into the deadly hydrogen arsenide (arsine, arseniuretted hydrogen), AsH3. As2O3 + 12 H = 2 AsH3 + 3 H2O. The AsH3 mixed with excess of H tends to escape and is burned to As2O3 and H2O, and thus is rendered comparatively harmless as it passes into the air. This is why the flame must be burning when the arsenic compound is introduced. 2 AsH3 + 6 O = As2O3 + 3 H2O.
In the combustion of AsH3, H burns at a lower point than As. The introduction of a cold body like porcelain cools the flame below the kindling-point of As, and this is deposited, while H burns, in exactly the same way as lamp- black was collected in Experiment 26.
210. Expert Analysis.—A modification of this experiment is employed by experts to test for AS2O3 poisoning. The organs.— stomach or liver—are cut into small pieces dissolved by nascent Cl, or HClO, made from KC1O3 and HCl, and the solution is introduced into a H generator, as above. AS2O3 preserves the tissues it comes in contact with, for a long time, and the test can be made years after death. All the chemicals must be pure, since As is found in small quantities in most ores, and the Zn, HCl, and H2SO4 of commerce are very likely to contain it. The above is called Marsh's test, and is so delicate that a mere trace of arsenic can be detected.
211. Properties and Occurrence.—As is a grayish white solid, of metallic luster, while a few of its characters are non-metallic. It is very widely distributed, being sometimes found native, and sometimes combined, as AsS, realgar, As2S8, orpiment, and FeAsS, arsenopyrite. Its chief source is the last, the fine powder of which is strongly heated, when As separates and sublimes. It has the odor of garlic, as may be observed by heating a little on charcoal with the blow-pipe.
212. Atomic Volume.—As is peculiar in that its atomic volume, so far as the volume can be determined, is only half that of the H atom. Its vapor density is 150, which gives 300 for the molecular weight, while its least combining or atomic weight is 75. 300, the molecular weight = 75, the atomic weight =4, the number of atoms in the molecule. All gaseous molecules being of the same size, represented by two squares, the atomic volume of As must be one-fourth of this size, represented by half of one square. Of what other element is this true? 213. Uses of As2O3.-Arsenic is used in shot-manufacture, for hardening the metal. Its most important compound is As2O3, arsenic trioxide, called also arsenious anhydride, arsenious acid, white arsenic, etc. So poisonous is this that enough could be piled on a one-cent piece to kill a dozen persons. Taken in too large quantities it acts as an emetic. The antidote is ferric hydrate Fe2(OH)6 and a mustard emetic, followed by oil or milk.
The vapor density of this compound shows that its symbol should be As4O6, but the improper one, As2O3, is likely to remain in use. Another oxide, As2O5, arsenic pentoxide, exists, but is less important. Show how the respective acid formulae are obtained from these anhydrides. See page 50.